Electrical power is transmitted via a power grid as alternating current from power generation facilities to industrial, commercial and residential customers. Generally, electrical power is generated by large electrical power generating stations. Recently, alternative sources of electrical power have been developed and connected to the power grid. Some of these sources, solar electric, for example, generate electrical power as direct current. In order for the electrical energy produced from these sources to be added to the grid, direct current is inverted, that is, changed from direct current to alternating current.
In addition, because the demand for electricity and some of the sources of electrical power, especially some of the newer sources, are variable, there is a need to match the demand to the source. Sources such as solar electric power, for example, generate electricity when the sun is shining, and wind power, when the wind is blowing. Similarly, electrical demand varies during the day, with the seasons, and evolves over time due to cyclical variations in the economy or structural usage growth such as increased use of electricity. Consequently, matching demand to capacity is a constant challenge. Inverting DC to AC is a growing part of that challenge.
Electric motors are used to produce torque to a shaft. The efficiency of an electric motor varies with load. Its efficiency, depending on the motor, will tend to peak as it approaches full load, at around 70%, and then decline slightly as it reaches full load; at loads lower than its peak, motor efficiency drops sharply. Motor efficiency is higher if its speed is constant.
It is known that inverting direct current using an inverter drive can be improved if smaller inverter drives are connected in electric series/parallel topologies. As demand grows, smaller inverters can be brought quickly on line one at a time, and all but the most recent one operated at full power. As demand falls, inverters are switched off one at a time so that, again, all of them but perhaps one of them are operated at full power. The overall efficiency of plural inverter drives is thus improved over one large drive, such as when demand is unsteady.
See U.S. Pat. No. 8,559,198, which discloses a high yield AC-AC power converter; U.S. Pat. No. 8,766,474, which discloses a smart micro-grid reconfigurable AC interface; and U.S. Pat. No. 9,159,487, which discloses a linear magnetic device, which are incorporated by reference herein in their entirety. See also chapter 17, p 455-p 486 of the Power Electronics Handbook by Muhammed H. Rashid, which is also incorporated herein by reference in its entirety, and which shows cascaded inverters with H-bridge rectifiers.
Because of the amount of electrical energy consumed worldwide and the interest in using alternative energy sources as part of the electric power grid, any improvement in energy production efficiency and reliability would yield important advantages.